Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of SUS316L-15CrMo Dissimilar Steel Joints Welded with Different Consumables
Literature Overview
The paper by Li Yajie, Liu Tao, Hong Lei, Hong Liang, and Huang Aiyi (2023, Materials in Mechanical Engineering, Vol. 47, Issue 10, pp. 43-49) investigates the TIG welding of SUS316L austenitic stainless steel to 15CrMo low-alloy steel using two different filler metals: ERNiCrMo-3 nickel-base alloy wire and ER309 stainless steel wire. The study was supported by the National Natural Science Foundation of China (Youth Fund, Grant No. 51601078) and the Jiangsu Provincial Government Overseas Scholarship Program (JS-2019-322). The research was conducted collaboratively among Jiangsu University of Technology, Jiangsu Better Fittings Co., Ltd., and the Taizhou Branch of the Jiangsu Provincial Special Equipment Safety Supervision and Inspection Institute.
This work is particularly significant for piping engineers dealing with dissimilar material transitions in high-temperature and high-pressure service environments, such as power plant steam lines and refinery transfer piping, where austenitic stainless steels are frequently joined to low-alloy steels to address corrosion resistance requirements at specific sections.
Core Technical Findings
Solidification Behavior Comparison
The two filler metals produce distinctly different solidification modes and microstructures in the weld metal:
| Parameter | ERNiCrMo-3 | ER309 |
|---|---|---|
| Solidification mode | Full austenitic | Austenite-ferrite mixed |
| Weld microstructure | Complete austenite | Austenite-ferrite mixture |
| Ferrite content | Near zero | Moderate delta-ferrite |
| Carbon diffusion on 15CrMo side | Suppressed | More pronounced |
The ERNiCrMo-3 filler produces a fully austenitic weld metal due to the high nickel and chromium content, which stabilizes the austenite phase across the entire solidification temperature range. In contrast, ER309 contains sufficient carbon and manganese to promote a mixed austenite-ferrite solidification, resulting in a dual-phase microstructure.
Mechanical Performance
| Property | ERNiCrMo-3 Joint | ER309 Joint |
|---|---|---|
| Tensile strength | Higher | Lower |
| Elongation after fracture | Higher | Lower |
| Peak hardness location | 15CrMo CGHAZ (both) | 15CrMo CGHAZ (both) |
| Weld hardness difference | ~30 HV higher than ER309 | Baseline |
| Laves phase presence | Yes | No |
The ERNiCrMo-3 joint achieves superior tensile strength and elongation, attributed to the suppression of carbon diffusion from the 15CrMo side into the weld metal. However, the presence of Laves phase (Fe2W-type intermetallic compound) in the ERNiCrMo-3 weld raises the hardness by approximately 30 HV, which warrants careful consideration regarding brittle fracture susceptibility under cyclic loading or thermal cycling conditions.
Carbon Diffusion and Intermetallic Formation
The key metallurgical insight is that the nickel-base filler acts as a diffusion barrier against carbon migration from the 15CrMo side. In ER309 welds, carbon diffuses more readily into the weld metal and the stainless steel side, potentially forming chromium carbides that reduce the corrosion resistance of the HAZ. The Laves phase observed in the ERNiCrMo-3 weld forms due to the enrichment of Mo and Cr at the weld/15CrMo interface during solidification, which is a common phenomenon in nickel-base welds to Cr-Mo steels.
Process Analysis and Engineering Implications
TIG Welding Process Parameters
The study employed conventional TIG (GTAW) welding with appropriate current density and travel speed. For dissimilar steel joints of this type, typical parameters include:
- Welding current: 80-120 A (DCEN)
- Travel speed: 3-6 cm/min
- Shielding gas: 100% Ar or 98% Ar + 2% N2
- Preheating: 150-250°C for 15CrMo side to reduce HAZ hardness
- Interpass temperature: Controlled below 250°C
Defect Risk Assessment
Using a simplified FMEA approach, the following defect risks are identified:
| Defect Type | Risk with ERNiCrMo-3 | Risk with ER309 | Mitigation |
|---|---|---|---|
| Carbon diffusion | Low | High | Use Ni-base filler |
| Laves phase brittleness | Moderate | Low | Post-weld heat treatment |
| Chromium carbide precipitation | Low | Moderate | Control cooling rate |
| HAZ cracking on 15CrMo side | Moderate | Moderate | Preheat and PWHT |
| Hot cracking in weld | Low | Low | Low travel speed |
Standards and Specification Considerations
For power plant applications, the selection of filler metal for dissimilar steel joints is governed by:
- ASME B31.1 Power Piping Code: Requires qualification of dissimilar material welds per Section IX
- ASTM A234 WP9: Specifies 15CrMo fittings
- ASTM A351 CF8M: Specifies SUS316L fittings
- API 5L: Provides guidance on welding consumables for line pipe
The use of ERNiCrMo-3 is consistent with AWS A5.11 specification for nickel-base electrode EBNiCrMo-3, which is designed for welding Cr-Mo steels and austenitic stainless steels. ER309 corresponds to AWS A5.9 specification and is the conventional choice for dissimilar steel welding per ASME Section IX.
Key Questions and Reflections
The 30 HV hardness increase due to Laves phase in the ERNiCrMo-3 weld raises an important engineering question: does this intermetallic compound compromise the long-term fatigue performance of the joint? Laves phase is known to be brittle and can serve as crack initiation sites under cyclic loading. In service conditions involving thermal cycling (such as power plant startup and shutdown cycles), this phase may undergo thermal fatigue cracking. A comprehensive assessment would require supplementary fatigue testing and possibly post-weld heat treatment to dissolve the Laves phase.
Furthermore, the study does not address the corrosion resistance of the weld metal and HAZ, which is critical when the SUS316L side is exposed to corrosive environments. The suppression of carbon diffusion is beneficial for preventing chromium carbide precipitation, but the effect of the nickel-base filler on the overall corrosion resistance of the joint, particularly under chloride-containing environments, warrants further investigation.
Study Insights and Engineering Recommendations
The findings clearly demonstrate that ERNiCrMo-3 is superior to ER309 for SUS316L/15CrMo dissimilar steel joints in terms of tensile strength, ductility, and carbon diffusion control. However, the Laves phase issue must be addressed through proper heat treatment. In engineering practice, the following recommendations are made:
- For critical dissimilar steel joints in power generation and petrochemical service, prefer ERNiCrMo-3 as the filler metal.
- Apply post-weld heat treatment (PWHT) at 620-650°C for 2-4 hours to dissolve Laves phase and relieve residual stresses.
- Implement preheating of the 15CrMo side to 200-250°C to reduce HAZ hardness and prevent cracking.
- Perform hardness mapping across the entire weld cross-section to verify the absence of excessive hardness gradients.
- Conduct metallographic examination to confirm the absence of intermetallic phases in the heat-affected zone.
The research provides valuable data for engineers selecting welding consumables for dissimilar material transitions. The carbon diffusion suppression capability of nickel-base fillers is a well-established principle, and this study quantitatively confirms its effectiveness in the SUS316L/15CrMo system. The Laves phase observation serves as a reminder that while nickel-base fillers offer excellent metallurgical compatibility, they are not without their own metallurgical challenges that must be managed through appropriate process control and post-weld treatment.